Evaluation of a Model for Estimating Infragravity Waves in Shallow Water: Accuracy and Suitability for Long-Term Climatologies

Abstract

With interest in developing shallow-water facilities on the increase, primarily for offloading LNG, there is also growing interest in infragravity waves. In particular, it is recognized that infragravity waves can have an important influence on the motions of tankers in shallow-water regions exposed to the open ocean, and therefore they need to be considered in the design and operation of the moorings and offloading facilities. Accordingly, there is a need to model infragravity waves both for design calculations and to be able to estimate the design criteria themselves. This paper is concerned with the later — setting infragravity wave design criteria, or more precisely on details involved in establishing an appropriate infragravity wave database from which design criteria can be estimated. The focus is on the accuracy of using the 1D linear Surf Beat model (IDSB) for estimating nearshore infragravity wave heights. The study has focused on field measurements made by U.S. Corp of Army Engineers Field Research Facility location at Duck on the east coast of the United States, and another location at Baja on the north west coast of Mexico. At the Duck location, the study involved data recorded in shallow water (8 meters water depth) with a pressure transducer array, while at the Baja location, data from a directional Waverider buoy with GPS are used. The “short wave” directional spectra from the measured data are used as input to the IDSB model, to compute the total infragravity response generated by the transformation of the grouped short waves through the surf zone including bound long waves, leaky waves and edge waves. The computed root mean square infragravity wave heights have been compared with measured infragravity waves at the respective sites, and assessment has been made of the accuracy of the predictions. The computed results show good agreement with the measured infragravity waves and provide confidence that the IDSB is a suitable tool for developing a long-term infragravity data set for developing design criteria.